Since an organic electroluminescence device is a self-luminescent device, it is luminous, excellent in visibility, and capable of giving clear display, as compared with a liquid crystal device, so that active investigations have been made. Recently, as an approach to increase luminous efficiency of a device, a device that generates phosphorescence using a phosphorescence-emitting substance, that is, utilizes luminescence from a triplet-excited state, has been developed. According to the theory of an excited state, in the case of using phosphorescence emission, there is expected remarkable increase in luminous efficiency such that the luminous efficiency of about 4 times the conventional fluorescence emission becomes possible.
In 1993, M. A. Baldo et al. in Princeton University realized an external quantum efficiency of 8% by the use of a phosphorescence-emitting device using an iridium complex.
Since a phosphorescence-emitting substance induces concentration quenching, the substance is supported by doping a charge-transporting compound generally called a host compound therewith. The emitting substance to be supported is called a guest compound. As the host compound, 4,4′-di(N-carbazolyl)biphenyl (hereinafter referred to as CBP):
has been used (see Non-Patent Document 1).    Non-Patent Document 1: Appl. Phys. Let., 75. 4 (1999)
However, since CBP has high crystallinity, it was pointed out to be poor in stability in a thin film state. For this reason, satisfactory device characteristics were not obtained in the case where heat resistance is required, for example, in the case of high luminance emission.
Therefore, 4,4′,4″-tri(N-carbazolyl)triphenylamine (hereinafter referred to as TCTA):
was proposed as a novel host compound and was confirmed to have luminous efficiency almost equal to that of CBP (see Non-Patent Document 2).    Non-Patent Document 2: Japan Society of Applied Physics, Organic Molecule Bioelectronics Section, Ninth Workshop, 17 (2001)
As the progress of the study on phosphorescence-emitting devices, elucidation of an energy transfer process between a phosphorescence-emitting device and a host compound was advanced and it was revealed that an excited triplet level of the host compounds should be higher than that of the phosphorescence-emitting device, so that there has been required a host material having an excited triplet level higher than that of CBP.
An external quantum efficiency of the phosphorescence-emitting device using, as a host compound in an emission layer, CBP doped with a blue phosphorescence-emitting substance Firpic:
is still 6%. As a reason therefor, the excited triplet level of CBP is as low as 2.56 eV as compared with the excited triplet level of Firpic of 2.62 eV, so that it is considered that confinement of the excited triplet excitons of Firpic is insufficient.
This is evidenced by the fact that photoluminescence intensity of a CBP film doped with Firpic shows temperature dependency (see Non-Patent Document 3).    Non-Patent Document 3: Japan Society of Applied Physics, Journal of Organic Molecule Bioelectronics Section, 14(1), 23 (2003)
When the host compound in the emission layer is CBP, the confinement of the excited triplet excitons of Firpic is insufficient, so that non-radiating transition of the temperature dependency occurs and photoluminescence intensity decreases at room temperature.
Thus, in order to enhance luminous efficiency of a phosphorescence-emitting device, there has been required a host compound for the emission layer, which completely confines the triplet excitons of the phosphorescence-emitting device.